1. Field of the Invention
This invention relates to catalytic conversion of C.sub.5.sup.+ hydrocarbon feeds or mixtures thereof (in or out of the presence of sulfur) to a C.sub.2 -C.sub.5 paraffin stream which is further thermally cracked to light olefins.
2. Description of the Prior Art
Zeolitic materials, both natural and synthetic, have been demonstrated in the past to have catalytic properties for various types of hydrocarbon conversions. Certain zeolitic materials are ordered, porous crystalline aluminosilicates having a definite crystalline structure within which there are a large number of smaller cavities which may be interconnected by a number of channels. These cavities and channels are uniform in size. Since the dimensions of these pores are such as to accept for absorption molecules of certain demensions while rejecting those of larger dimensions, these materials have come to be known as "molecular sieves" and are utilized in a variety of ways to take advantage of these properties.
Such molecular sieves, both natural and synthetic, include a wide variety of positive ion-containing cyrstalline aluminosilicates. These aluminosilicates can be described as a rigid three-dimensional framework of SiO.sub.4 and AlO.sub.4 in whch the tetrahedra are cross-linked by the sharing of oxygen atoms whereby the ratio of the total aluminum and silicon atoms to oxygen is 1:2. The electrovalence of the tetrahedra containing aluminum is balanced by the inclusion in the crystal of a cation, for example, an alkali metal or an alkaline earth metal cation. This can be expressed wherein the ratio of aluminum to the number of various cations, such as Ca/2, Sr/2, Na, K or Li is equal to unity. One type of cation may be exchanged either entirely or partially by another type of cation utilizing ion exchange techniques in a conventional manner. By means of such cation exchange, it has been possible to vary the properties of a given aluminosilicate by suitable selection of the cation. The spaces between the tetrahedra are occupied by molecules of water prior to dehydration.
Prior art techniques have resulted in the formation of a great variety of synthetic aluminosilicates. These aluminosilicates have come to be designated by letter or other convenient symbols, as illustrated by zeolite A (U.S. Pat. No. 2,882,243), zeolite X (U.S. Pat. No. 2,882,244), zeolite Y (U.S. Pat. No. 3,130,007), zeolite ZK-5 (U.S. Pat. No. 3,247,195), zeolite ZK-4 (U.S. Pat. No. 3,314,752), zeolite ZSM-5 (U.S. Pat. No. 3,702,886), zeolite ZSM-11 (U.S. Pat. No.3,832,449), zeolite ZSM -12 (U.S. Pat. No. 3,832,449), and zeolite ZSM-20 (U.S. Pat. No. 3,972,983) merely to name a few.
A crysalline aluminosilicate zeolite well known in the art is faujasite. The ZSM-20 zeolite for use in the present invention resembles faujasite in certain aspects of structure, but has a notably higher silica/alumina ratio than faujasite.
It is known from U.S. Pat. No. 3,598,724 that C.sub.3 -C.sub.4 paraffins can be produced from hydrocarbons boiling in the range from 100.degree. F. to 550.degree. F. by hydrocracking over a catalyst comprising mordenite mixed with a nickel/tin catalyst supported on an amorphous inorganic oxide.
Similar patents such as U.S. Pat. No. 3,385,782 suggest the hydrocracking of high boiling hydrocarbon fractions into C.sub.1 -C.sub.4 paraffins.
These prior art process, as well as U.S. Pat. No. 3,718,575 are primarily concerned with the production of LPG or liquified petroleum gas (C.sub.3 -C.sub.4) useful as fuels and thus were interested mainly in producing higher yields of C.sub.3 -C.sub.4 rather than C.sub.2 -C.sub.3.